Recent years have seen climate changes and natural disasters likely due to global warming significantly affecting agricultural production, dwelling environments, and energy consumption. Global warming is believed to be caused by increased greenhouse gases in the atmosphere accompanying the expansion of human activities, such as carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons. Among these greenhouse gases, carbon dioxide in the atmosphere is considered to be the primary cause, and taking measures to reduce emissions of carbon dioxide into the atmosphere is a worldwide agenda.
The sources of carbon dioxide include thermal power plants using fuels such as coal, heavy oil, and natural gas, ironworks blast furnaces for reducing iron oxide with coke, ironworks converters for burning carbon contained in pig iron to manufacture steel, factory boilers, and cement plant kilns, as well as transport equipment including automobiles, marine vessels, and aircraft using fuels such as gasoline, heavy oil, and light oil. Except for transport equipment, the sources above are stationary facilities where it is thus easy to take measures to reduce emissions of carbon dioxide into the atmosphere.
Several methods for separating and capturing carbon dioxide from gases emitted from the sources listed above are known.
For example, a method for absorbing carbon dioxide by bringing a carbon dioxide-containing gas into contact with an aqueous solution of an alkanolamine in an absorption tower is known.
Examples of known alkanolamines include monoethanolamine, diethanolamine, triethanolamine, methyl diethanolamine, diisopropanolamine, and diglycolamine, and of these, monoethanolamine is widely used.
The use of aqueous solutions of these alkanolamines as an absorbing liquid for carbon dioxide, however, requires the use of expensive corrosion-resistant steel or requires lowering the concentration of the amine in the absorbing liquids because primary amines such as monoethanolamine severely corrode materials of equipment. The absorbed carbon dioxide is typically desorbed and regenerated by heating the absorbing liquid to about 120° C. in a regeneration tower, but this method ends up consuming a large amount of energy in capturing carbon dioxide per unit weight because the use of the alkanolamines is unsatisfactory in terms of the amount of absorbed carbon dioxide in the absorption tower and the amount of desorbed carbon dioxide in the regeneration tower.
In an age in which reducing CO2 emissions and saving energy and natural resources are being sought, large energy consumption in separating and capturing carbon dioxide has been a major constraint on practical use of the technology, and techniques for separating and capturing carbon dioxide with less energy need to be developed.
As an example of prior art techniques for separating and capturing carbon dioxide using less energy, PTL 1, for example, discloses a method for removing carbon dioxide from a combustion exhaust gas by bringing an aqueous solution of a specific hindered amine into contact with a combustion exhaust gas at atmospheric pressure. The Examples of PTL 1 disclose N-methylaminoethanol and N-ethylaminoethanol as hindered amines, and also other amines, such as 2-isopropylaminoethanol, which is not used in the Examples though.
PTL 2 discloses an absorbing liquid that contains a mixture of multiple alkanolamines and that achieves the highest performance taking advantage of the characteristics of each amine, and a method for absorbing carbon dioxide.
These absorbing liquids disclosed in PTL 1 and 2, however, cannot sufficiently reduce the energy required for separating and capturing carbon dioxide. In PTL 3, studies were conducted on the use of a non-aqueous organic compound such as an alcohol instead of an aqueous liquid, which uses water with a large specific heat capacity, as a solvent. Because the use of an alcohol, for example, instead of water lowers the specific heat, and carbon dioxide is once converted to unstable alkyl carbonate in the steps of separating and capturing carbon dioxide, the use of an alcohol is expected to improve low-temperature desorption. Nonetheless, an absorbing liquid of such a composition exhibits extremely low CO2 absorption efficiency and requires absorption of carbon dioxide to be performed at low temperatures in the range of 20° C. to 25° C., meaning that extra energy for cooling in absorption is necessary.
PTL 4 and 5 propose an absorbing liquid of a two-phase separation system. After having absorbed an acidic compound such as carbon dioxide, the absorbing liquid separates into a phase rich in the acidic compound and a phase poor in the acidic compound. From these phases, the phase rich in the acidic compound is separated with, for example, a decantation equipment, and only from the phase rich in the acidic compound, the acidic compound is desorbed in an effort to reduce the amount of the absorbing liquid to be heated and energy required in desorption. However, a significant amount of the acidic compound also remains in the phase poor in the acidic compound, and the capture efficiency is in fact unsatisfactory.